YAG (yttrium aluminum garnet) has been used as a laser for laser machining and medical applications. The YAG laser which is a solid-state laser, however, has a low electrical-to-optical conversion efficiency. This is due to the fact that the Xe lamp and the flash lamp of the conventional YAG laser used for exciting the solid-state laser medium is very low in luminous efficiency. Also, the luminous spectrum band of the above-mentioned lamp is so wide that only a small portion of the luminous energy can be used for the solid-state laser excitation. Normally, therefore, a bulky device and a large amount of cooling water are required.
On the other hand, the semiconductor laser (“LD”) has a high luminous efficiency and is so compact that no bulky cooling unit is required. In recent years, the price of the high-power semiconductor laser has conspicuously decreased. This semiconductor laser can be desirably used also for the laser machining. The beam quality of the semiconductor laser, however, is generally so low and the output power level of the single-stripe semiconductor laser is limited. Therefore, the semiconductor laser is generally can be used directly for laser machining only in the limited fields of application.
A multi-stripe semiconductor laser, in which 10 to 100 stripes of junction for radiating the laser beams are linearly arranged to provide a dashed-line light source, is known as a high-power laser.
A linear-array semiconductor laser of about 50 W in CW (continuous wave oscillation) output with the junction of the semiconductor laser aligned one-dimensionally is available. In the multi-stripe array semiconductor laser, as shown in FIG. 1, for example, tens to several tens of stripes 100 μm to 200 μm wide with each facet making up an emitter are arranged at regular intervals within a plane 1 cm wide.
Several layers of this linear array semiconductor are stacked into a two-dimensional array as shown in FIG. 1, thereby making it possible to easily increase the output. This two-dimensional array semiconductor laser is referred to as the stack array semiconductor laser.
As described above, one semiconductor laser device provides a light source with line segments arranged in a two-dimensional array for emitting laser beams in the number of n multiplied by ten to several tens, where n is the number of stacked layers. Also, a high-power semiconductor laser such as the quasi-CW semiconductor laser provides a light source, in which substantially continuous linear light sources in the number of stacked layers are arranged comprising a multiplicity of closely-arranged emitters in which each emitted light beam is mixed with a light beam immediately after emission from an adjoining emitter.
In order to use a multi-stripe array semiconductor laser in laser machining and medical applications, certain ways should be devised to concentrate high-level energy in a narrow area. Each striped light beam is emitted from a flat light source, and the beam divergent angle has a component φ perpendicular to the junction which is as large as about 40 to 50 degrees, while the component θ parallel to the junction is about 10 degrees. The width of the light source includes the vertical portion as small as not more than 1 μm and the horizontal portion as wide as 100 μm to 200 μm as described above.
The above-mentioned characteristics of the semiconductor laser are such that in the case where the light emitted from the semiconductor laser is focused using a lens, the vertical component can be focused easily, while the horizontal component is not easily focused into a small spot due to a large width and a smaller divergent angle than the vertical component.
The light emitted from a stack array laser diode can be focused into a linear line by use of a cylindrical lens for each linear array, but cannot be easily focused in a dot spot.
A comparatively efficient focusing can be achieved, on the other hand, by a method in which micro lenses are arranged in one-to-one correspondence with stripes, whereby each beam from stripe is collimated and focused thereby to superimpose a plurality of beams. The spot diameter of the reduced beam is the product of the width of the light source and the magnification (f2/f1) determined by the ratio of the distance between the focusing lens and the beam spot (i.e. the focal length f2 of the focusing lens) to the distance between the semiconductor laser stripes and the micro lens (i.e. the focal length f1 of the micro lens).
Thus, the long diameter ω1 (horizontal component) of the beam spot is equal to the product (ω0·f2/f1) of the width of the stripe (ω0: 100 μm to 200 μm) and the aforementioned magnification. As to the vertical component, on the other hand, a spot diameter does not become large by multiplying the same ratio (f2/f1) due to a very small width of the light source (not more than 1 μm). Considering the transverse focusing of the stripe, therefore, the micro lens is desirably arranged as distant from the stripe as possible to secure a small beam spot and a high light intensity.
Due to a large divergence angle of the vertical component of the stripe light beam and hence a large radiation energy leaking out of the lens aperture, however, it is difficult to arrange the micro lens as described above. In an alternative idea, therefore, the vertical and horizontal components are focused by different cylindrical lenses, so that the lens for focusing the vertical component is arranged in one-to-one correspondence with each linear array semiconductor laser in proximity to the stripes, while the lens for focusing the horizontal component is arranged in one-to-one relation with each equivalent stripe group of the stack layer distant from the stripes.
As an example, a stack array LD is considered in which 12 stripes each 1 μm thick and 200 μm wide are spaced 800 μm apart, and the linear arrays are stacked in several layers. The horizontal component of the emitted beam from the stripe has a beam divergent angle of 10 degrees. Therefore, the beam emitted from the adjoining stripes are superposed with each other at the distance of 3.4 mm from the light-emitting facet of the stripe. In the case where a lens is placed after the superposition, part of the light beams is converted into a light ray at an angle to the lens axis and is focused at a point different from the focal point of the focusing lens, thereby reducing the system efficiency.
In order to collimate the light radiated from each stripe group using a micro cylindrical lens array, therefore, a lens (having a focal length f1 of not larger than 3.4 mm) is required to be placed at a position as near as not more than 3.4 mm. The diameter of the focused spot is unavoidably large, if determined as the product of the stripe width and the magnification (f2/f1) dependent on the combination with the focal length f2 of the focusing lens.
In conventional arrangement and methods, as described above, the laser beam emitting from the stack array LD providing a light source comprising two-dimensional array of line segments cannot be easily concentrated with high density in a small area.
Also, according to the end surface excitation method using the optical excitation from the direction of the optical axis of the LD pumped solid-state laser, a highly efficient single transversal-mode oscillation can be realized by matching the space excited by the semiconductor laser output light with the oscillation mode volume of the solid-state laser.
In the stack array semiconductor laser oscillation element with the junction of the semiconductor laser arranged two-dimensionally, the output of about 1 kW is obtained and sufficiently usable for the laser machining. If this stack array laser beam is directly focused using an optical system and can be reduced to a sufficiently small spot, the output of the semiconductor laser should be usable for the laser machining.
In the case where the light emitted from the stack array semiconductor laser source is focused using a lens, however, the vertical component can be focused with comparative ease, whereas the horizontal component is difficult to focus to a minuscule spot due to the large width of the light source.
An attempt to use the stack array semiconductor laser as an excitation source for the solid-state laser leads to the result that the end surface excitation method having a high excitation efficiency cannot be employed because a plurality of beams cannot be focused to a single spot using a conventional lens system due to the fact that the array width is about 1 cm. Therefore, only the side pumping method is utilized.